1. Field of the Invention
This invention relates to ZnSe type pin and avalanche photodiodes having a sensitivity range from ultraviolet rays to blue light. A pin photodiode (pin-PD or PD) is an optoelectronic device having functions of making a strong electric field at a pn junction by applying reverse bias, receiving input light, generating pairs of holes and electrons, accelerating the holes and electrons by the reverse bias and producing photocurrent by the holes and electrons in proportion to power of the input light. An avalanche photodiode (APD) further amplifies flows of carries (electrons and holes) by inducing an avalanche of carriers by a pre-applied large reverse bias.
This application claims the priority of Japanese Patent Application No. 2002-244795 filed on Aug. 26, 2002, which is incorporated herein by reference.
A photodiode has sensitivity for light of a bandgap wavelength λg corresponding to a bandgap Eg of a material of a light receiving layer (active layer) and for light of wavelengths shorter than the bandgap wavelength λg. Here, the bandgap wavelength is defined by an equation of λg=hc/Eg (h is Planck's constant, c is light velocity in vacuum). A photodiode cannot detect light of wavelengths far shorter than the bandgap wavelength λg. Different kinds of photodiodes should be utilized for different wavelength bands of object light. Most prevalent photodiodes are silicon photodiodes (Si-PD) at present. Silicon has a bandgap Eg=1.1 eV and a bandgap wavelength λg=1.1 μm. Silicon photodiodes (Si-PDs) have a wide sensitivity range from visible to near-infrared light (λ≦1.1 μm). Germanium photodiodes (Ge-PDs) are employed for detecting light of wavelengths longer than the sensitivity range of Si-PDs. Germanium (Ge) has a bandgap Eg=0.67 eV and a bandgap wavelength λg=1.8 μm. Ge-PDs can detect infrared light of wavelengths up to 1600 nm. Si-PDs and Ge-PDs can cover a far wide wavelength range from visible light to infrared light. Besides Si-PDs and Ge-PDs, indium phosphide photodiodes (InP-PDs) which have an InGaAs sensing layer piled upon an InP substrate are utilized for sensing light of a 1.55 μm band and a 1.3 μm band prevailing in optical communications. The three types of photodiodes—Si-PDs, Ge-PDs and InP-PDs—are prevalent. But, these three kinds of photodiodes cover only the wavelength range of the visible light and infrared light. None of the three types of photodiodes has sensitivity for blue, violet and ultraviolet rays. No photodiodes are still available for detecting light of shorter wavelengths. Even silicon photodiodes have poor sensitivity for violet and near-ultraviolet rays.
Reception of violet light and ultraviolet rays requires new photodiodes having an active layer made of a material with a wide bandgap corresponding to object light colors of short wavelengths, blue, violet or ultraviolet.
Gallium nitride (GaN) and zinc selenide (ZnSe) are well known as a wide bandgap material. Gallium nitride (GaN) is an excellent material for blue-light emitting devices (light emitting diodes (LED) and laser diodes (LDs)). GaN has overcome ZnSe as a material for light emitting devices. Gallium nitride (GaN), however, is bad for a material of photodetectors. No good GaN substrate single crystal is obtainable yet at present. If we tried to make a GaN photodiode, we would heteroepitaxially pile GaN-layers on a sapphire substrate. GaN layers grown on a sapphire substrate have many dislocations and defects. An on-sapphire GaN photodiode would be annoyed at poor sensitivity and large dark current due to large defect density.
Zinc selenide (ZnSe) was defeated by GaN in the race of making light producing devices (LEDs or LDs). ZnSe is still promising as a material for making light receiving devices (PDs or APDS) instead of light producing devices. ZnSe has a bandgap wavelength λg=460 nm. λg=460 nm gives ZnSe a possibility of becoming a favorable material for photodiodes for detecting blue light and violet light.
One purpose of the present invention is to provide a ZnSe type photodiode enabling us to detect blue, violet and ultraviolet rays.
2. Description of Related Art
At present, materials which enable us to obtain large single crystal substrates are silicon (Si), germanium (Ge), indium phosphide (InP), gallium arsenide (GaAs) and gallium phosphide (GaP). No large good bulk single crystal of zinc selenide can be grown yet. ZnSe type devices should be built on foreign material substrates. N-type gallium arsenide (GaAs) single crystals have been used as substrates for making ZnSe type devices, since the lattice constant of ZnSe is close to that of GaAs.
Electrons have far higher mobility than holes in GaAs. In general, n-type GaAs wafers have been dominantly used for making optoelectronic devices, for example, photodiodes, light emitting diodes and laser diodes.
On the contrary, p-type GaAs wafers have a poor demand due to small hole mobility. Thus, “GaAs wafers” have indicated n-type GaAs wafers.
We imagine an on-n-GaAs ZnSe photodiode now. If a ZnSe-photodiode were made by piling n-, i- and p-ZnSe-type material layers on an n-GaAs substrate, an imaginary PD would have a metallic n-electrode, an n-GaAs single crystal substrate, an n-ZnSe buffer layer, an n-ZnSe layer, an i-ZnSe layer, a p-ZnSe layer and a metallic p-electrode in series from bottom to top. But, such a ZnSe-PD would not operate with high efficiency.
A drawback is difficulty of making p-type ZnSe. A more serious drawback in the on-n-GaAs ZnSe-PD is that it is impossible to form a p-electrode in ohmic contact with a p-ZnSe layer. The wide bandgap ZnSe prohibits us from forming a metallic p-electrode on p-ZnSe. Zinc telluride (ZnTe) which has a narrower bandgap than ZnSe allows us to make a good p-ZnTe layer and make a good p-electrode upon the p-ZnTe layer. For overcoming the difficulties of the production of p-layers and the formation of p-electrodes on ZnSe, a superlattice of p-ZnSe/ZnTe having a p-ZnTe layer on top is grown on a p-ZnSe layer. Then, a metallic p-electrode is made upon the top p-ZnTe layer. The p-electrode makes an ohmic contact with the undercoating p-ZnTe layer. The metallic p-electrode should be made into an annular shape or a small dotted shape for allowing incidence light to go via the p-electrode into the p-ZnTe layer.
A virtual pn-junction type or pin-junction type ZnSe-photodiode could be made upon an n-type GaAs substrate. An n-metallic electrode as a cathode would be formed upon the n-GaAs substrate. If a reverse bias were applied between the metallic p-electrode (anode) and the bottom n-electrode (cathode), a depletion layer (strong electric field area) would be formed at the i-layer or at the pn-junction. If light beams went into the photodiode and arrived at the depletion layer, photocurrent would be induced. Blue light or violet light could be detected by the photocurrent.
However, such an on-n-GaAs ZnSe photodiode would have the following drawbacks. Incidence light going via an anode (p-electrode) into the photodiode would pass p-type ZnTe layers which are included in a p-ZnTe contact layer and the (ZnTe/ZnSe) superlattice (MQW: multiquantum well). ZnTe which has a bandgap narrower than ZnSe would absorb the incidence light which has energy higher than the ZnTe bandgap. The absorption of the incidence light by the ZnTe layers is one of serious weak points of the on-n-GaAs ZnSe photodiode. A decrease of the light absorption would require thinning of the ZnTe layers. But, the p-ZnTe contact layer would be indispensable for ohmic contact with the p-electrode and the p-ZnTe multiquantum layers would be also necessary for forming the superlattice (MQW) structure. The absorption caused by the p-ZnTe layers would lower external quantum efficiency. Absorption loss induced by the p-ZnTe layers would be more conspicuous for near-ultraviolet rays.
One purpose of the present invention is to provide an on-GaAs ZnSe photodiode with low absorption loss due to ZnTe. Another purpose of the present invention is to provide an on-GaAs ZnSe photodiode with high external quantum efficiency.